Phobos

Phobos is the larger and closer of the two small moons that circle Mars, and one of the strangest satellites in the solar system. It is not a sphere but a battered, lopsided lump of dark rock about 27 by 22 by 18 km across — small enough that its feeble gravity, less than a hundredth of Earth's, would let a determined astronaut throw a rock clean off it into orbit around Mars. Named for the Greek personification of fear, the son and companion of the war god Ares, it is a fitting attendant to the red planet.

What makes Phobos remarkable is not its size but its fate. It orbits closer to its planet than any other moon in the solar system, races around Mars faster than Mars itself turns, and is slowly falling. In a few tens of millions of years it will be torn apart — an ending that makes this doomed little moon one of the most scientifically interesting bodies within reach of a spacecraft.

Discovery and naming

Phobos was found on 17 August 1877 by the American astronomer Asaph Hall, working at the U.S. Naval Observatory in Washington during a close approach of Mars. Hall had spotted the fainter, outer moon Deimos a few nights earlier and had nearly abandoned the search in frustration; by his own account, his wife Angeline Stickney urged him to keep looking, and he found Phobos six nights after its sibling. The two moons take their names from the twin sons of Ares who personified fear (Phobos) and dread (Deimos) in Greek myth.

Hall's wife is honoured on the moon itself: Stickney, the enormous crater that dominates one end of Phobos, carries her maiden name. Many of the moon's other features are named after people connected to its discovery or after characters and places from Jonathan Swift's Gulliver's Travels, whose fictional astronomers of Laputa had, satirically, given Mars two moons more than a century before Hall actually found them.

A captured asteroid, or Martian debris?

Phobos is extremely dark, reflecting only about 7% of the light that hits it, and its surface closely matches carbon-rich, primitive material like that seen in carbonaceous chondrite meteorites and D-type asteroids from the outer asteroid belt. That resemblance long suggested the simplest story: Phobos is a wandering asteroid that strayed too close to Mars and was captured. The problem is that its orbit is nearly circular and lies almost exactly in Mars's equatorial plane, which is hard to achieve by capture alone.

The competing idea is that Phobos and Deimos coalesced from a ring of debris blasted into orbit by a giant impact on Mars — the same broad mechanism thought to have formed Earth's Moon. Its low density of about 1.88 g/cm³ and an estimated internal porosity near 30% point to a loosely bound 'rubble pile' rather than a solid rock, consistent with material that re-accreted from smaller fragments. Settling this question is a central goal of upcoming exploration.

Stickney and the mysterious grooves

The single most striking feature on Phobos is Stickney crater, roughly 9 km across on a body only about 22 km wide. The impact that carved it was very nearly enough to shatter the moon; fine dust has since drifted down its inner walls, leaving dark streaks where boulders have tumbled toward the crater floor.

Radiating across much of Phobos are long, parallel grooves — typically less than 30 m deep, 100 to 200 m wide, and up to 20 km long. For decades these striations were assumed to be fractures left by the Stickney impact. More recent modelling suggests instead that they were carved by chains of boulders thrown out by the impact, rolling and bouncing across the surface, while other researchers argue some grooves are stretch marks from the tidal stresses that are slowly pulling the moon apart.

The fastest, closest moon — and a doomed one

Phobos orbits just 9,376 km from the centre of Mars, only about 6,000 km above the surface — closer to its planet than any other known moon. It completes a full circuit in just 7 hours and 39 minutes, roughly three times every Martian day. Because it circles faster than Mars rotates, an observer on the Martian surface would see Phobos rise in the west and set in the east, crossing the sky twice a day, moving fast enough that its phases are visible to the naked eye. Phobos is tidally locked, keeping the same battered face turned toward Mars at all times.

That low, fast orbit is also lethal. Tidal interaction with Mars is dragging Phobos inward at roughly 2 metres per century. In an estimated 30 to 50 million years it will either strike the Martian surface or, more likely, cross the Roche limit and be torn apart by tides into a temporary ring of rubble encircling the planet — a rare case of a moon whose destruction we can already forecast.

Spacecraft that have visited

Phobos has been photographed by nearly every mission sent to Mars. Mariner 9 returned the first detailed images in 1971, the Viking orbiters captured sharp close-ups in the late 1970s, and modern eyes — the Mars Reconnaissance Orbiter's HiRISE camera and ESA's Mars Express, which has made repeated close flybys since 2004 — have mapped its grooves and Stickney in fine detail. The Soviet Union dedicated two probes, Phobos 1 and Phobos 2, to the moon in 1988; Phobos 2 returned some data before contact was lost just before its planned close approach. Russia's Fobos-Grunt sample-return attempt failed to leave Earth orbit in 2011.

The next chapter belongs to Japan. JAXA's Martian Moons eXploration (MMX) mission is scheduled to launch in late 2026, orbit Mars, land on Phobos to scoop up more than 10 grams of surface material, and return that sample to Earth around 2031 — the first samples ever brought back from the Martian system. If Phobos really is Martian debris, some of that material may even be ancient Mars itself, launched into space by the impact that made the moon.